Relationship between vacuum level, positive pressure, and negative pressure, as well as conversion relationships for vacuum units

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I. Simply put, these three concepts correspond respectively to the thin, normal, and dense states of a gas. 　　Atmospheric pressure: refers to one atmosphere, that is, the gas pressure generated by the atmosphere in which we live on a daily basis. One standard atmosphere is 101325 Pa (Pascal, a common unit of pressure). 100,000 Pa = 100 KPa; therefore, \"one standard atmosphere\" is also commonly expressed as 100 KPa or 101 KPa. Due to differences in geographical location, altitude, temperature, and so on, the actual atmospheric pressure in various places differs from the standard atmospheric pressure. However, for simplification purposes, it is sometimes possible to assume that standard pressure equals 100 KPa. Negative pressure refers to a gas state in which the pressure is lower than standard pressure, which is what we commonly call a \"vacuum\". For example, when drinking from a straw, there is negative pressure inside the straw; similarly, there is negative pressure inside the suction cups used for hanging things. 　　Positive pressure: refers to a gas state with a pressure higher than normal atmospheric pressure. For example, when inflating bicycle or car tires, the positive pressure is generated at the outlet end of the pump or inflator. II. In various applications such as scientific research, bioengineering, automatic control, environmental protection, and water treatment, gas sampling, gas circulation, and the adsorption of substances are often required, and vacuum pumps are used for these purposes. Its main parameters include vacuum level, flow rate, etc. (1) “Vacuum level” generally refers to the ultimate pressure that can be achieved when the pump is in operation; in other words, it denotes the degree of thinness of the gas remaining in a sealed container after the gas inside has been pumped out. 　　In industry, there are two ways in which limit pressure can be expressed. One is “absolute pressure,” where “absolute vacuum” – an absolute vacuum that can only be achieved in theory, with no matter present – is used as the zero point; the values indicated are all positive. The smaller this value, the closer it is to absolute vacuum, meaning the degree of vacuum is higher. For example, there is a “high-vacuum” micro vacuum pump. Its maximum pressure is 10 KPa (0.01 MPa), which constitutes a very high level of vacuum for micro vacuum pumps. 　　The other type is “relative pressure,” which uses atmospheric pressure as the reference point; values below atmospheric pressure are expressed as negative numbers, which is why it is called “negative pressure.” The larger the absolute value of this negative value, the higher the vacuum level. 　　The standard and most scientific approach used in the international vacuum industry is to use \"absolute pressure\" for designation; however, since measuring relative pressure is simpler and the corresponding measuring instruments are widely available (such as ordinary vacuum gauges which are all relative pressure gauges), \"relative pressure\" is commonly used in China for this purpose. 　　Relationship between the two: Relative pressure = Absolute pressure – Local atmospheric pressure. 　　For example, the absolute pressure of VCH1028 is 10 Kpa; its relative pressure = 10 – 100 = -90 Kpa (-0.09 MPa). (II) In fields such as scientific research, laboratories, and medicine, gas pressurization is often used, for example, to inflate containers that are already under positive pressure, or when there is high resistance within a system and a pump is needed to overcome this resistance in order to supply gas. At this point, the pump needs to be able to generate a positive pressure higher than atmospheric pressure, which is usually expressed as “relative pressure”. High-pressure micro air pumps and micro vacuum pumps can generate a positive pressure of >100 Kpa (0.1 MPa). They belong to the category of dry vacuum pumps; they do not require vacuum pump oil or lubricants, and they do not contaminate the working medium. They can operate continuously for 24 hours, and their intake and exhaust ports can become clogged – making them particularly suitable for such applications. Comprehensive example: (Not entirely precise, just to illustrate the relationship among the three.) Assume that the gas pressure inside a sealed container is at atmospheric pressure, meaning there are 100 gas molecules inside. Using VCH1028 with a negative pressure of -90 Kpa, 90 molecules can be removed, leaving 10 molecules; in this case, the negative pressure inside the container is -90 Kpa. If PH2506B is used, only 75 molecules can be removed, leaving 25 molecules, and the corresponding negative pressure inside the container is -75 Kpa. 　　If PCF5015N is used to inflate this container, there will ultimately be 200 gas molecules inside it; expressed in absolute pressure, this is 200 Kpa, while in terms of relative pressure (positive pressure), it is 100 Kpa. 　　The term \"vacuum level\" commonly used in the international vacuum industry refers to \"ultimate vacuum, absolute vacuum level, and absolute pressure.\" However, \"relative vacuum level\" (relative pressure, gauge pressure of a vacuum gauge, negative pressure) is more widely used due to its simple measurement methods and the widespread availability of measuring instruments. 　　Conversion formula: Relative vacuum degree = Standard atmospheric pressure – Absolute vacuum degree. For example, if the absolute vacuum degree is 80 KPa, then its relative vacuum degree is approximately 100 – 80 = 20 KPa; thus, it will be displayed as -0.02 MPa on a relative vacuum gauge. 　　Appendix: Pressure unit conversions: 1 Pa = 1.02×10-5 Kgf/cm2 = 1×10-5 bar = 0.01 mbar = 9.87×10-6 atm = 7.5×10-3 torr = 4.01×10-3 inH2O = 7.5×10-3 mmHg = 1.45×10-4 PSI